Introduction: Multi Jet Fusion is a powder-bed fusion process that builds nylon parts layer by layer, and that category is what makes a service description readable.
Most people meet Multi Jet Fusion (MJF) through a quote form rather than a textbook. They upload a CAD file, see a material called PA12, a build envelope of 380 × 284 × 380 mm, a tolerance of ±0.3 mm or ±0.3%, and a note about escape holes, and they have to decide whether any of it matters for their part. The concepts come out of order that way. This explanation climbs the ladder in the right sequence: first where MJF sits among additive manufacturing categories, then how the powder-bed fusing sequence actually forms a nylon part, and finally why a service description works as a process definition — a set of conditions a design has to respect.
Where Multi Jet Fusion Sits Among Additive Manufacturing Categories
Additive manufacturing means building a part from 3D model data by adding material layer by layer instead of cutting it away, a definition TWI sets out in its overview of the field. Loughborough University's additive manufacturing research group sorts the whole field into seven process categories: binder jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination, and vat photopolymerization. That classification is the useful first step, because it tells you what family a process belongs to before you look at any specific machine. MJF belongs to powder bed fusion. Everything about the process follows from that placement: a bed of polymer powder, a recoating blade or roller that spreads each layer, and a thermal event that fuses the powder together where the part should be. Within powder bed fusion, MJF is the variant that does not rely on a laser tracing each contour. A print carriage moves across the bed and deposits liquids onto the powder, and infrared lamps then pass over the whole layer. NIST frames additive manufacturing research around measurement, repeatability, and process understanding, which is exactly the level at which the category matters to a designer: powder bed fusion processes are chosen for polymer parts that need to come out of a loose powder cake, nested together in a build, rather than being grown on a platform or extruded through a nozzle.
How the Powder-Bed Fusing Sequence Forms a Functional Nylon Part
The sequence repeats one geometry-building move: spread powder, print liquids, apply heat, drop the bed, spread again. A recoater lays a thin layer of nylon powder across the build area. The carriage jets two fluids onto selected regions. Infrared lamps sweep the layer, and the treated powder fuses into a solid slice of the part while untreated powder stays loose. The build platform then drops by one layer thickness, fresh powder is spread on top, and the cycle runs again until the whole cake is built. After the build, the cake cools, the part is dug out, depowdered, and often bead-blasted before any optional dyeing or vapor smoothing.
1. Fusing Agents and Detailing Agents Control Where Each Layer Bonds
The two fluids do different jobs, and their split explains a lot about the feature quality MJF can hold. The fusing agent is printed where the part should be solid; it absorbs infrared energy efficiently, so the powder under it heats past its melting point and bonds to the layer below. The detailing agent is printed at the boundary of that region and slightly beyond it. It works the other way, suppressing fusion, which keeps edges crisp and stops molten material from bleeding into surrounding powder. Because both fluids are deposited by a print head rather than decided by a single beam path, the pattern of solid and non-solid regions on each layer is defined in the same motion across the whole bed. That is the mechanism behind fine feature definition in MJF nylon parts.
2. Powder Support Removes Separate Supports but Not Escape Holes
The unfused powder surrounding the part acts as its support, so an MJF build has no separate support structures to design, generate, or cut off. Overhangs and internal features sit in powder rather than in mid-air. That advantage has a practical consequence that catches people out: the powder has to get back out. Any enclosed cavity, blind channel, or trapped volume needs escape holes sized so loose powder can leave during depowdering and blasting. A design that would print cleanly in a filament process with breakaway supports can still trap powder in MJF if it has no escape path. AIHFABS notes that its design review checks escape holes and even walls, which is the right place for that thinking — before the file is quoted, not after the part arrives full of nylon.
Why MJF Service Descriptions Work as Process Definitions
A process definition describes what the equipment does and the conditions under which a part comes out as intended. A service description describes what is on offer: upload CAD, choose a material and finish, receive a 3D printing service quote, get parts shipped. The two are layered, not competing. When AIHFABS positions MJF for production-grade nylon and TPU parts with PA12 as the primary material, natural gray, dyed black, and vapor smoothing as finishes, and TPU or glass-bead filled PA12 available by quote, those are process facts arranged as an offer. The envelope of 380 × 284 × 380 mm is a planning box: a part inside it can usually be nested somewhere in a build, while what actually lands in your quote also depends on nest density and the finish you chose. Tolerance of ±0.3 mm or ±0.3% applies to typical MJF geometry, so long, thin, or heavily post-processed features are worth discussing with the person quoting. The reason this distinction is worth learning is that it changes how you read a service description. An engineer reviewing a powder-bed process for the first time tends to look for the support question, tick it off, and move on — then notice later that escape holes still appear in the design notes. Reading the description as a process definition makes that obvious in advance: powder supports the part, and powder must still be removed, so escape paths are part of the design, not an optional courtesy. The same habit covers materials and finishes. PA12 is the default nylon for functional parts; TPU is the route to gaskets, grips, and flexible connectors, and its durometer is confirmed at quote stage. A TPU 3D printing service offer is meaningful precisely because the process definition tells you what the material can and cannot do in a powder bed. On demand manufacturing service pages that list conditional limits are more useful than ones that promise outcomes, because the limits are what a design has to work with.
Conclusion
MJF is one powder-bed fusion process inside a seven-category additive manufacturing field, and the powder-bed part of that placement explains almost everything a designer needs: fusing and detailing agents define where each layer bonds, powder supports the part, and escape holes stay mandatory because that powder has to leave. Read a service description as a process definition and the numbers stop looking arbitrary. The build envelope becomes a planning box, the tolerance becomes a starting condition for typical geometry, and the finish names become states you can compare. AIHFABS publishes those process facts alongside its MJF service, which is the sensible way to see a supplier's offer: the definition first, the commercial details in the quote.
FAQ
Q: What type of additive manufacturing process is Multi Jet Fusion?
A: Multi Jet Fusion is a powder bed fusion process. A recoater spreads polymer powder across a build area, a print carriage deposits fusing and detailing agents onto each layer, and infrared lamps fuse the treated regions. That places MJF in the same broad additive manufacturing category as other powder bed fusion systems, and it builds parts by adding material layer by layer rather than removing it.
Q: Why does MJF not need separate support structures?
A: The unfused powder surrounding the part holds it in place throughout the build. Overhangs, internal channels, and free-standing features sit in loose nylon powder rather than in mid-air, so there is no support geometry to design, generate, or cut away afterwards. The build cake effectively becomes the fixture, and parts can be nested closely together in the same build volume.
Q: Why are escape holes still needed in MJF parts if powder supports the build?
A: The powder that supports the part also fills enclosed cavities, blind holes, and internal channels. That material has to come out during depowdering and blasting, and an enclosed volume with no opening keeps it trapped. Escape holes give the loose powder an exit path, which is why they remain a core design rule even though the process has no separate support structures to remove.
Sources / References
What is Additive Manufacturing? (Definition & Types) - TWI
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